Many types of bearings comprise a lubricant such as grease or oil for reducing wear and fatigue of the bearing. In particular, wear and fatigue of the rolling elements and/or the raceways of the bearing are reduced by the provision of a lubricant inside the bearing.
Some lubricated bearings require frequent, periodic maintenance to prevent premature failure. Other lubricated bearings require little maintenance, these bearings being described by terms such as “sealed bearing” or “sealed for life”. A sealed bearing comprises a seal such as a rubber lip which avoids or minimizes as best as possible leakage of the lubricant from inside the bearing to the ambient.
In practice, however, a certain leakage of lubricant out of the bearing has often to be accepted. This has the consequence that the amount of lubricant inside the bearing decreases over the time. This in return makes a refill of lubricant necessary from time to time.
A highly relevant question regarding timing and planning the moment to refill lubricant into the bearing is the question how large the amount of lubricant that exists inside the bearing actually is. Unfortunately, it is quite difficult to determine the amount of lubricant inside a bearing. In other words, it is difficult to determine the filling level of lubricant inside a bearing.
In the related art, determination of the filling level of lubricant has to be carried out manually by a service personnel examining the bearing. For large industrial type bearings, this is often done endoscopically. This, however, means that specialized service personnel is possibly needed. Furthermore, frequent visits for inspection of the bearing in order to determine the filling level of lubricant inside the bearing is costly.
Another approach in the related art is to ignore the actual filling level of lubricant inside a bearing, and instead to carry out a refill of lubricant in predefined intervals. This has the disadvantage that suboptimally timed service visits for refilling the bearing is made. Particularly for bearings of a wind turbine, with the wind turbine being located at a site which is difficult to access, unnecessary service visits for refilling lubricant into a bearing are costly. Sites which are difficult to access are, for example, offshore sites or sites at remote and/or mountainous regions.
Finally note that a delayed refill of lubricant to a bearing may cause considerable costs due to wear. Thus, a bearing which is operated at critically low filling levels of lubricant may cause considerable repair costs.